Computers commonly use disc drives for memory storage purposes. Disc drives include a stack of one or more magnetic discs that rotate and are accessed using a head or read-write transducer. Typically, a high speed motor such as a spindle motor is used to rotate the discs.
In conventional spindle motors, stators have been made by laminating together stamped pieces of steel. These stamped pieces of steel are generally circular in nature, but also have “poles” extending either inwardly or outwardly, depending on whether the rotor is on the inside or surrounds the stator. The stamped pieces are laminated together and then coated with insulation. Wire is then wound around the poles to form stator windings.
An example of a conventional spindle motor 1 is shown in FIG. 1. The motor 1 includes a base 2 which is usually made from die cast aluminum, a stator 4, a shaft 6, bearings 7 and a disc support member 8, also referred to as a hub. A magnet 3 and flux return ring 5 are attached to the disc support member 8. The stator 4 is separated from the base 2 using an insulator (not shown) and attached to the base 2 using a glue. Distinct structures are formed in the base 2 and the disc support member 8 to accommodate the bearings 7. One end of the shaft 6 is inserted into the bearing 7 positioned in the base 2 and the other end of the shaft 6 is placed in the bearing 7 located in the hub 8. A separate electrical connector 9 may also be inserted into the base 2.
Each of these parts must be fixed at predefined tolerances with respect to one another. Accuracy in these tolerances can significantly enhance motor performance.
In operation, the disc stack is placed upon the hub. The stator windings are selectively energized and interact with the permanent magnet to cause a defined rotation of the hub. As hub 8 rotates, the head engages in reading or writing activities based upon instructions from the CPU in the computer.
Manufacturers of disc drives are constantly seeking to improve the speed with which data can be accessed. To an extent, this speed depends upon the efficiency of the spindle motor, as existing magneto-resistive head technology is capable of accessing data at a rate greater than the speed offered by the highest speed spindle motor currently in production. The efficiency of the spindle motor is dependent upon the dimensional consistency or tolerances between the various components of the motor. Greater dimensional consistency between components leads to a smaller gap between the stator 4 and the magnet 3, producing more force, which provides more torque and enables faster acceleration and higher rotational speeds.
The conventional method of forming stators has a number of drawbacks. First, most steel is manufactured in rolled sheets and thus has a grain orientation. The grain orientation has an effect on the magnetic flux properties of the steel. In circular stamped pieces of steel, the grain orientation differs at different points around the circle. Compared from the radius line of the circle, the grain orientation is sometimes aligned along the radius, sometimes transverse to it, and mostly at a varying angle to the radius. The un-aligned grain structure of conventional stators causes the magnetic flux values to differ in parts of the stator, and thus the motor does not have consistent and uniform torque properties as it rotates.
Another drawback with using circular steel pieces is that, especially for inward facing poles, it has been difficult to wind the wire windings tightly because of the cramped space to work inside of the laminated stator core. The cramped working space creates a lower limit on the size of the stator and thus the motor. The limited working space also results in a low packing density of wire. The packing density of wire coiled around the poles affects the amount of power generated by the motor. Increasing packing density increases the power and thus the efficiency of the spindle motor.
An important factor in motor design is to reduce stack up tolerances in the motor. Stack up tolerances reduce the overall dimensional consistency between the components. Stack up tolerances refer to the sum of the variation of all the tolerances of all the parts, as well as the overall tolerance that relates to the alignment of the parts relative to one another. One source of stack up tolerances is from the circular stator body. Generally, the thickness of rolled steel is not uniform across the width of the roll. Sometimes the edges are thicker or thinner than the center. In a stator made from circular stamped pieces, the thicknesses of individual laminations are thus different from one side to the other. When stacked together, this creates a stack up tolerance problem. Furthermore, the circular stampings leave a lot of wasted steel that is removed and must be recycled or discarded.
Another important factor in motor design is the lowering of the operating temperature of the motor. Increased motor temperature affects the electrical efficiency of the motor and bearing life. As temperature increases, resistive loses in wire increase, thereby reducing total motor power. Furthermore, the Arhennius equation predicts that the failure rate of an electrical device is exponentially related to its operating temperature. The frictional heat generated by bearings increases with speed. Also, as bearings get hot they expand, and the bearing cages get stressed and may deflect, causing non-uniform rotation, reducing bearing life. This non-uniform rotation causes a further problem of limiting the ability of the servo system controlling the read/write heads to follow data tracks on the magnetic media. One drawback with existing motor designs is their limited effective dissipation of the heat, and difficulty in incorporating heat sinks to aid in heat dissipation. In addition, in current motors the operating temperatures generally increase as the size of the motor is decreased.
Manufacturers have established strict requirements on the outgassing of materials that are used inside a hard disc drive. These requirements are intended to reduce the emission of materials onto the magnetic media or heads during the operation of the drive. Of primary concern are glues used to attach components together, varnish used to insulate wire, and epoxy used to protect steel laminations from oxidation.
In addition to such outgassed materials, airborne particulate in a drive may lead to head damage. Also, airborne particulates in the disc drive could interfere with signal transfer between the read/write head and the media. To reduce the effects of potential airborne particulate, hard drives are manufactured to exacting clean room standards and air filters are installed inside of the drive to reduce the contamination levels during operation.
An example of a spindle motor is shown in U.S. Pat. No. 5,694,268 (Dunfield et al.) (incorporated herein by reference). Referring to FIG. 5 of this patent, a stator of the spindle motor is encapsulated with an overmold 42. The overmolded stator 40 contains openings through which mounting pins 44 may be inserted for attaching the stator 200 to a base. U.S. Pat. No. 5,672,972 (Viskochil) (incorporated herein by reference) also discloses a spindle motor having an overmolded stator. One drawback with the stators described in these patents is this difficulty in winding wire on the poles. Another drawback is the height of the lamination stacks. Further, the overmolds shown in these patents are not effective in dissipating heat or dampening some vibrations generated by energizing the stator windings.
U.S. Pat. No. 5,806,169 (Trago) (incorporated herein by reference) discloses a method of fabricating an injection molded motor assembly. However, neither the Trago design nor the other prior art designs address the problems of winding wire, variation in the thickness of steel used to make the stator cores and the non-uniform grain structure in the steel compared to the magnetic flux in the stator during operation of the motor.
Some of these problems have been addressed by motor manufacturing methods in which individual stator arc segments are made and wound with wire to form poles, and these segments are then assembled to form a complete stator. While this process allows for higher packing density, it has several drawbacks. Somehow the individual segments have to be assembled and held in place to form the stator. In addition, the individual wires of the different poles have to be connected together for the poles that are of the same phase. These numerous wires tend to get in the way during the assembly process, slowing down the manufacturing process.
U.S. Pat. No. 6,049,153 to Nishiyama describes the use of crimping or welding to attach segments together. This process deforms the steel and reduces the level of magnetic flux produced by the laminations. The process also requires numerous wire interconnections when the poles are wound as discrete components, and it does not offer improvements in wire routing.
U.S. Pat. No. 5,729,072 to Hirano describes the use of welding or an adhesive to hold the segments together. A disadvantage of this approach is that the stator poles must be handled as separate elements during stator construction. This requires complicated assembly equipment and a slow manufacturing process.
U.S. Pat. No. 6,265,804 to Nitta describes the use of plastic insulation in combination with segmented stators. This approach does not improve on the problem of how to assemble and hold the individual segments in place, nor does it aid in connecting the various wires.
U.S. Pat. No. 6,167,610 to Nakahara describes a method of making a rotary motor where a length of steel strip has thin portions between blocks of pole teeth. Wire is wound on the pole teeth while the steel strip is straight. Later the thin sections are bent to allow the poles to form a stator. One problem with this design is that when the thin portions are bent, the stress on the steel reduces the flux capacity of the connecting steel, forming the back iron. Also, the stamping of such a length of steel strip would be expensive and result in large amount of scrap. Thus, a need exists for a method of making motors overcoming the aforementioned problems.